1. Field of the Invention
The field of this invention resides in the area of manhole frames and more particularly relates to adjustable manhole frames which are easily raisable to new pavement heights.
2. Description of the Prior Art
The standard practice in manhole installations is to supply precast concrete manholes which are assembled at the job site. These manholes extend from the depth of the pipe up to its top cone which is disposed about 12 inches below the road surface. This 12-inch space is for the installation of a manhole frame and cover which assemblies vary in height from 4 to 10 inches. The void between the bottom of the manhole frame's outwardly extending bottom flange and the cone of the concrete manhole is filled with either brick, mortar, brick and mortar, cement block and mortar, or precast concrete rings mortared in.
After installation of the frame, available fill material consisting of gravel or a combination of gravel and dirt is deposited around the exterior of the manhole cone and manhole frame above its outwardly extending bottom flange, and the fill is then compacted with whatever means is available, leaving 2 inches or more on top for the final paving material. Invariably, settling occurs within a short period of time resulting in a separation and cracking of the final paving around the frame. This settling process causes potholes to develop that require constant patching. Over a period of time seepage of surface liquids such as water, gasoline, oil, and in cold climate areas, water carrying salt causes deterioration of the support base under the manhole frame. In areas of severe deep frost penetration, entire sections of the frame and its base can be lifted above the road surface. A great number of these structures have to thus be repaired which repair necessitates extensive and expensive digging out of the entire manhole frame and the subsequent rebuilding of its base.
At the present the vast majority of manhole frame members are of the non-adjustable type, therefore any needed repair necessitates digging them entirely out of the ground to raise, lower, repair or replace worn or broken frames.
It has long been desirable to have a simple and easy way of providing an adjustable manhole frame member to avoid the necessity of digging up the entire manhole frame out of the ground which project entails a great deal of labor with air compressor equipment and jack hammers to cut through the pavement and to loosen the compacted fill with the accompanying great expense and traffic dusruption.
In an attempt to raise manholes without the necessity of digging out the entire structure, one prior technology utilized ring members for placement upon existing frame cover seats into which the covers are replaced. These ring members have limited use as their minimum overall height is greater than the height of the thinner layer of pavement used today. Also, if they are not carefully installed or do not properly fit the cover seat and cavity, these ring members can be dislodged and cause traffic accidents.
Many adjustable manhole frames are disclosed in the prior art. One is the "Preko" adjustable manhole frame disclosed in U.S. Pat. No. 3,858,998 which includes a second frame within an outer frame with screw adjustment means to raise or lower the inner second frame member in relation to the outer frame member so that if the road level is raised, one turns the screws on the manhole, and the inner frame will raise or lower the manhole cover to the desired height. These "Preko" adjustable manhole frames have not gained widespread usage in the industry because of their higher initial cost due to the complexity of machining that must be done to create them and to the inherent fear that the screw elements might jam and fail to operate after the frame has been installed for a long period of time.
Another adjustable type is the "self-level", a United Kingdom invention disclosed in U.S. Pat. No. 4,174,183. This frame is installed during paving operations and requires hot pavement as well as the use of a pavement roller for compression of the paving material and the positioning of the frame. A special exterior concrete ring is also required which is mortared to the base of the manhole. Height adjustments for this frame are limited to shallow height adjustments, and when the frame is raised, a void can be created that could trap water which, if frozen, could force the unit up into the traveled way. Further, unless care is taken to properly fill and compact around the frame's exterior, surface and underground liquids could undermine and/or deteriorate the frame support.
A further development in adjustable manhole frames is disclosed in my U.S. Pat. No. 4,536,103 which provides an improved adjustable manhole frame and method of construction utilizing a formed concrete base or base of brick or cement mortar that supports the frame of the invention in the pavement. As disclosed in this patent, the height and angle of the specially designed inner frame are determined by the method of installation which sets the height and angle of the formed concrete base as the frame is installed.